列表 4. BookPK.java，我们实体 Bean 的主键类

package il.co.lerner.book;

import java.io.Serializable;

public class BookPK implements java.io.Serializable
{
    public int id;
    public BookPK () { }

    public BookPK(int value)
    {
        id = value;
    }

    public boolean equals(Object obj)
    {
        if (obj == null || !(obj instanceof BookPK))
            return false;

        if (((BookPK)obj).id == id)
            return true;

        return false;
    }

    public int hashCode ()
    {
        return id;
    }


    public String toString()
    {
        return String.valueOf(id);
    }
}